Bile acids are biomolecules which are required for the digestion and absorption of fats, fatty acids and hydrophobic vitamins. A bile acid which is found, in humans, in small amounts only is ursodeoxycholic acid (UDCA). It has recently gained great therapeutic importance in the dissolution of cholesterol-comprising gallstones. This compound is produced industrially in tonne quantity by chemical or enzymatic steps. An important precursor for the synthesis of UDCA is 12-ketoursodeoxycholic acid, which can be converted into UDCA by a Wolff-Kishner reduction. A route, described in the literature, for the synthesis of 12-ketoursodeoxycholic acid starts with cholic acid (3α,7α,12α-trihydroxy-5β-cholanic acid), which may be prepared by two oxidative steps which are catalyzed by 7α- and 12α-HSDHs, and one reductive step, catalyzed by a 7β-HSDH (Bovara R et al. (1996) A new enzymatic route to the synthesis of 12-ketoursodeoxycholic acid. Biotechnol. Lett. 18:305-308; Monti D et al. (2009) One-pot multienzymatic synthesis of 12-ketoursodeoxycholic acid: Subtle cofactor specificities rule the reaction equilibria of five biocatalysts working in a row. Adv. Synth. Catal. 351:1303-1311). A further route starts with 7-ketolithocholic acid, which may be converted into UDCA by stereoselectively reducing the 7-keto group; this step, too, is advantageously carried out with enzymatic catalysis, catalyzed by a 7β-HSDH (Higashi S et al. (1979) Conversion of 7-ketolithocholic acid to ursodeoxycholic acid by human intestinal anaerobic microorganisms: Interchangeability of chenodeoxycholic acid and ursodeoxycholic acid. Gastroenterologia Japonica 14:417-424; Liu L et al. (2011) Identification, cloning, heterologous expression, and characterization of a NADPH-dependent 7 beta-hydroxysteroid dehydrogenase from Collinsella aerofaciens. Appl. Microbiol. Biotechnol. 90:127-135.). A further advantageous synthetic route starts with dehydrocholic acid (DHCA), which may be converted into 12-ketoursodeoxycholic acid by two reductive steps; these two steps may be catalyzed by two stereoselective HSDHs (3α- and 7β-HSDHs) (Carrea G et al. (1992) Enzymatic synthesis of 12-ketoursodeoxycholic acid from dehydrocholic acid in a membrane reactor. Biotechnol. Lett. 14:1131-1135; Liu L et al. (2013) One-step synthesis of 12-ketoursodeoxycholic acid from dehydrocholic acid using a multienzymatic system. Appl. Microbiol. Biotechnol. 97:633-639).
The enzyme from C. aerofaciens has proved to be a very suitable 7β-HSDH. The gene sequence of this enzyme from C. aerofaciens is now known, so that firstly the enzyme can be made available recombinantly after cloning; secondly, it is possible to generate mutants of this enzyme by protein engineering methods and therefore optionally to find more advantageous enzyme variants, as may be the case.
The active substances ursodeoxycholic acid (UDCA) and its diastereomer chenodesoxycholic acid (CDCA) have, inter alia, been employed for many years as medicaments for the treatment of gallstone complaints. The two compounds differ merely by the configuration of the hydroxyl group on C atom 7 (UDCA: β-configuration, CDCA: α-configuration). To prepare UDCA, a variety of processes are described in the prior art, and these processes are carried out by purely chemical means or else as a combination of chemical and enzymatic process steps. The starting point is in each case cholic acid (CA), or CDCA, which is prepared starting from cholic acid.
Thus, the traditional chemical method for the preparation of UDCA can be shown diagrammatically as follows:

A severe disadvantage is, inter alia, the following: since the chemical oxidation is not selective, the carboxyl group and the 3α- and 7α-hydroxy group must be protected by esterification.
An alternative chemical/enzymatic process based on the use of the enzyme 12α-hydroxysteroid dehydrogenase (12α-HSDH) can be shown as follows and is described for example in PCT/EP2009/002190 of the present applicant.

Here, the 12α-HSDH oxidizes CA selectively to give 12-keto-CDCA. The two protective steps which are required by the traditional chemical method can be dispensed with here.
Furthermore, Monti, D., et al., (One-Pot Multienzymatic Synthesis of 12-Ketoursodeoxycholic Acid: Subtle Cofactor Specificities Rule the Reaction Equilibria of Five Biocatalysts Working in a Row. Advanced Synthesis & Catalysis, 2009) disclose an alternative enzymatic/chemical process, which can be shown diagrammatically as follows:

The CA is oxidized first by the 7α-HSDH enzyme from Bacteroides fragilis ATCC 25285 (Zhu, D., et al., Enzymatic enantioselective reduction of-ketoesters by a thermostable 7-hydroxysteroid dehydrogenase from Bacteroides fragilis. Tetrahedron, 2006. 62(18): p. 4535-4539) and 12α-HSDH to give 7,12-diketo-LCA. These two enzymes are in each case NADH-dependent. After the reduction by 733-HSDH (NADPH-dependent) from Clostridium absonum ATCC 27555 (DSM 599) (MacDonald, I. A. and P. D. Roach, Bile induction of 7 alpha-and 7 beta-hydroxysteroid dehydrogenases in Clostridium absonum. Biochim Biophys Acta, 1981. 665(2): p. 262-9), 12-keto-UDCA results. A Wolff-Kishner reduction gives the end product. The disadvantage of this process is that a complete conversion is not possible due to the equilibrium situation of the catalyzed reaction, and that two different enzymes must be employed in the first step of the reaction, which makes the process more expensive. Lactate dehydrogenase (LDH; for regenerating NAD+) and glucose dehydrogenase (GlcDH or GDH, for regenerating NADPH) are employed for cofactor regeneration. The disadvantage of the cofactor regeneration used in that reaction is that the co-product which forms can only be removed with great difficulty from the reaction mixture, so that the reaction equilibrium cannot be influenced positively, which brings about incomplete conversion of the starting material.
A 7β-HSDH from the strain Collinsella aerofaciens ATCC 25986 (DSM 3979; previously Eubacterium aerofaciens) was described in 1982 by Hirano and Masuda (Hirano, S. and N. Masuda, Characterization of NADP-dependent 7 beta-hydroxysteroid dehydrogenases from Peptostreptococcus productus and Eubacterium aerofaciens. Appl Environ Microbiol, 1982. 43(5): p. 1057-63). Sequence information for that enzyme was not disclosed. The molecular weight as determined by gel filtration amounted to 45 000 Da (cf. Hirano, page 1059, left-hand column). Furthermore, the reduction of the 7-oxo group to the 7β-hydroxy group could not be observed for said enzyme (cf. Hirano, page 1061, discussion, 1st paragraph). A person skilled in the art can therefore see that the enzyme described by Hirano et al. is not suitable for catalyzing the reduction of dehydrocholic acid (DHCA) in the 7-position to give 3,12-diketo-7β-CA.
The applicant's earlier international patent application PCT/EP2010/068576 describes a novel 7β-HSDH from Collinsella aerofaciens ATCC 25986, which has, inter alia, a molecular weight (as determined by SDS gel electrophoresis) of approximately 28-32 kDa, a molecular weight (as determined by gel filtration, under non-denaturing conditions, such as, in particular without SDS): of approximately 53 to 60 kDa, and the ability of stereoselectively reducing the 7-carbonyl group of 7-keto-LCA to a 7β-hydroxy group.
Furthermore, PCT/EP2010/068576 provides a process for the preparation of UDCA which can be shown diagrammatically as follows:

Thus, CA is oxidized in a simple manner via the traditional chemical route. The DHCA is reduced by the enzyme pair 7β-HSDH and 3α-HSDH, individually one after the other or else in one pot, to give 12-keto-UDCA. In combination with Wolff-Kishner reduction, UDCA can thus be synthesized in only three steps, starting from CA. While the 7β-HSDH enzyme is dependent on the cofactor NADPH, the 3α-HSDH enzyme requires the cofactor NADH. The availability of enzyme pairs which are dependent on the same cofactor or with extended dependence (for example on the cofactors NADH and NADPH) would be advantageous because it could simplify cofactor regeneration.
WO 2012/080504 describes novel 7β-HSDH mutants from C. aerofaciens in the sequence region of the amino acid residues 36 to 42 of the C. aerofaciens sequence, and biocatalytic processes for the preparation of UDCA, in particular also novel whole-cell processes.
WO 2011/147957 describes novel knock-out strains which are particularly suitable for the preparation of UDCA since it has been possible to switch off the undesired 7alpha-HSDH enzyme activity in targeted fashion.
The problem of the present invention is the provision of further improved 7β-HSDHs. In particular, it was intended to provide enzyme mutants which can be employed even more advantageously for the enzymatic or microbial preparation of UDCA via the stereospecific reduction of DHCA in the 7-position to give 3,12-diketo-7β-CA, and which have in particular an improved activity for substrate and/or cofactor, and/or of a reduced substrate inhibition and/or altered cofactor utilization (increased, modified specificity or widened dependency).